1. Field of the Invention
This invention relates to temperature sensing devices, and more particularly to such devices using the temperature sensitive properties of transistors. The present invention also relates to integrated circuit devices designed to generate a controlled output current over a wide temperature range and subject to substrate leakage.
2. Description of the Prior Art
A variety of temperature devices are known. Standard devices, such as thermocouples, thermistors, or RTD's, all have limitations which prevent them from being easily or widely used. Thermocouples require accurate cold junction compensation, some form of linearization, and produce a low level voltage output which is subject to electrical noise interference. Resistance thermometers and thermistors have a non-linear characteristic which requires careful compensation and a wide dynamic electrical range. In addition, making a good resistance measurement demands an accurate voltage source, low level precision current sensing, and careful lead compensation. These devices also require strict attention to lead wire material connections. Initial calibration on all these devices is also a problem especially when field replacement is necessary.
Another known form of temperature sensing is based upon the capability of a transistor to generate a base-to-emitter voltage proportional to absolute temperature, accurate over a wide temperature range. One previous attempt to use this property has employed means for amplifying and buffering the voltage signal and providing necessary support circuitry, such as a voltage regulator, on the same integrated circuit chip. This arrangement, however, is useful over only a limited temperature range, and because of large power consumption requirements is subject to self heating effects which interfere with the temperature sensing function.
Another approach to using the linear Vbe versus temperature property of a transistor to form a temperature sensing device is disclosed in U.S. Pat. No. 3,940,760 to Brokaw. This device, illustrated herein in FIG. 1, generates an output current I.sub.T varying with absolute temperature by means of first and second transistors Q1 and Q2 operated at a constant ratio of emitter current densities and having their bases interconnected and the difference between their Vbe's impressed across a resistor R. In the transistors Q1 and Q2, the equation for emitter current density is given by: EQU Je = Js e.sup.qVbe/kT
where Js is the saturation current density, q is the charge in coulombs of an electron, k is Boltzman's constant, and T is absolute temperature in degrees Kelvin. For two transistors at current densities J1 and J2: ##EQU1## The difference voltage V.sub.T is given by: ##EQU2##
Thus, if Jel/Je2 is a constant r, not equal to one, then ##EQU3##
In the circuit of FIG. 1, a constant ratio of emitter current densities is achieved by providing the first and second transistors Q1 and Q2 with emitter conductive areas of different sizes, and by using additional transistors Q3 and Q4 connected to the collectors of transistors Q1 and Q2, together with diode connections across transistors Q2 and Q3, in order to supply currents through transistors Q1 and Q2. Assuming that the transistor collector currents are dependent only on Vbe and base currents are zero (i.e., .beta. = infinity), then equal currents Ic1 = Ic2 are forced through transistors Q1 and Q2. Assuming the emitter conductive areas of transistors Q1 and Q2 are in a ratio r, the ratio of emitter current densities also will be r, and the difference voltage V.sub.T is directly proportional to absolute temperature. The voltage V.sub.T appears across resistor R and determines the level of current flowing through transistor Q1. The output current I.sub.T drawn by both sides of the circuit is ##EQU4##
If the resistor R has a zero temperature coefficient, then I.sub.T is also directly proportional to absolute temperature, and appropriate selection of the emitter ratio r and resistance R will provide an output current accurately related to temperature with a predetermined constant of proportionality, useful for absolute temperature sensing purposes.
While the circuit of FIG. 1 provides basically advantageous characteristics for use as a temperature sensing device, it would be desirable to effect performance improvements for certain applications, e.g. for extremely accurate temperature sensing, especially over wide temperature ranges. In the circuit of FIG. 1, high accuracy depends upon the achievement of equal currents through the first and second transistors Q1 and Q2 which equality, however, cannot precisely be achieved due to the finite .beta. of real components, and due to the Early effect error arising from the diode connected transistors (for a larger collector-base voltage, the base region becomes narrower because the collector depletion layer widens and the reduction of base thickness permits more emitter current to reach the collector for a fixed emitter-base voltage such that both emitter and collector currents are increased). In addition, the output I.sub.T of the circuit of FIG. 1 is a variable current, and when the circuit is realized in integrated circuit form, it has been found that over large temperature ranges, substrate leakages (i.e., across isolation pockets) produce leakage currents which affect the linearity of the output.